Posted by amy on .
I need to create a 100ml buffer with a pH of 4.00 using 0.100M Benzoic acid (pKa = 4.20)and 0.180M sodium benzoate. What volume of each material do I need to make this buffer? I think I need to use pH=pKa + log(A/HA)but am not sure if that is correct. Any help would be appreciated.

biochem 
DrBob222,
Yes, the HendersonHasselbalch equation is what you use.
If x = mL sodium benzoate (base)
then 100x = mL benzoic acid(acid).
4.00 = 4.20 + log(0.18x)/[(100x)*0.1]
and solve for x 
biochem 
amy,
I think that is where I keep getting stuck, with the basic math. I have 0.631 = .180x/100.1x and don't know where to go from there. Math was a long time ago for me :)

biochem 
DrBob222,
0.631 = (0.180x)/[(100x)*0.100]
0.631*[(100x)*.100] = 0.180x
multiply 0.631*0.100 first to obtain 0.0631, then
0.0631*(100x) = 0.180x
6.310.0631x = 0.180x
6.31 = 0.0631+0.180x
6.31 = 0.2431x
x = 6.31/0.2431 = 25.96 mL
100x = 74.04 mL
These should be rounded to 26.0 mL and 74.0 mL.
You should check this by multiplying 26.0x0.180 = ?? and 74.0 x 0.100 mL = ??, substitute into the HH equation and see if you arrive at a pH of 4.00. 
biochem 
amy,
thank you so much. I just recently went back to school and havent had chemistry in more than 4 years and math has been even longer. I really appreciate the help :)